Recently, light emitting elements such as LEDs (Light Emitting Diodes) are used as a plane light source such as a backlight of a liquid crystal display. In a case where LEDs are used as a plane light source, normally, a plurality of LEDs are connected with each other in series to form an LED row, and a plurality of LED rows are connected with each other in parallel to form an LED matrix. A transistor is inserted in series in each LED row. A current flowing through the LED row is made constant by the transistor.
However, individual LEDs have variations in forward voltage drop. Consequently, there are variations in a collector-emitter voltage to be applied on the transistor inserted in each LED row. Accordingly, in a case of an LED matrix used as a plane light source, it is necessary to set a voltage to be applied commonly on the LED rows to be higher in order that the transistor inserted in each LED row makes a current constant in an active region, i.e. a voltage applied on the transistor inserted in each LED row (collector-emitter voltage) surely exceeds a saturation voltage.
In order to deal with such a problem, Patent Literature 1 discloses a light emitting device including a light emitting element drive circuit which applies as small voltage as possible to individual LED rows.
With reference to FIG. 6, the following description will discuss a light emitting device described in Patent Literature 1. FIG. 6 is a circuit diagram of the light emitting device described in Patent Literature 1. As illustrated in FIG. 6, the light emitting device includes an LED drive circuit 50 and an LED light source circuit 60.
As illustrated in FIG. 6, the LED light source circuit 60 includes a switching circuit 61 and an LED circuit 62. The LED circuit 62 is obtained by connecting four LED rows 621 to 624 in parallel, and corresponds to the aforementioned LED matrix. The switching circuit 61 increases or decreases a voltage to be applied commonly on the four LED rows 621 to 624. As illustrated in FIG. 6, the switching circuit 61 includes a power source voltage 611, a coil 612, a transistor 613, a diode 614, and a capacitor 615.
As illustrated in FIG. 6, the LED drive circuit 50 includes a constant current driver 51, an LED drive control section 52, and a DCDC controller 53. The constant current driver 51 includes (i) transistors 511 to 514 which are connected in series with the LED rows 621 to 624, respectively, and (ii) resistors 515 to 518 which are connected in series with the LED rows 621 to 624, respectively. As illustrated in FIG. 6, an emitter terminal of the transistor 514 is connected with one input terminal of an operational amplifier 521, and a base terminal of the transistor 514 is connected with an output terminal of the operational amplifier 521. Consequently, as long as a voltage equal to or larger than a saturation voltage is applied across a collector and an emitter of the transistor 514 and the transistor 514 operates in an active region, a current flowing through the LED row 624 is made constant. The transistors 511 to 513 are connected with operational amplifiers (not shown) similarly, and constitute constant current circuits which make currents flowing through the LED rows 621 to 623 constant, respectively.
The LED drive control section 52 includes the operational amplifier 521, cooperates with the constant current driver 51 to realize a constant current function, and controls the DCDC controller 53 so that the minimum value of a collector-emitter voltage of each of the transistors 511 to 514 is kept a little larger than a saturation voltage of the transistors 511 to 514. Specifically, a comparison circuit 527 generates a difference Δ between the minimum value Vmin of the collector voltages of the transistors 511 to 514 and a reference voltage Vref supplied from a constant voltage source 522, and the DCDC controller 53 controls a direct current voltage to be supplied to a switching circuit 61 so that the difference Δ V is 0.
With this configuration, the light emitting device of Patent Literature 1 drives LEDs while preventing excessive application of voltages on the LEDs.